As a material of optical elements to be used in optical systems of various cameras such as cameras, film integrated type cameras and video cameras, an optical glass or an optical transparent resin is used. Optical glasses are excellent in heat resistance, transparency, size stability, chemical resistance, etc., and there are various optical glasses with different refractive indexes or Abbe numbers. However, optical glasses have problems of high material costs, bad molding processability and low productivity. In particular, significantly advanced techniques and high costs are required for processing for obtaining an aspherical lens to be used for aberration correction, and this is a major obstacle from a practical viewpoint.
Contrary to the above-described optical glasses, advantageously, optical lenses made of optical transparent resins, particularly thermoplastic transparent resins can be mass-produced by injection molding, and in addition, an aspherical lens can be easily produced therefrom. Such optical lenses are currently used as camera lenses. Examples of optical transparent resins include a polycarbonate made of bisphenol A, a polymethyl methacrylate and an amorphous polyolefin.
In general, in optical systems of cameras, aberration is corrected by combining a plurality of concave lenses and convex lenses. Specifically, chromatic aberration is synthetically corrected by combining convex lenses having chromatic aberration with concave lenses having chromatic aberration whose sign is opposite to that of the chromatic aberration of the convex lenses. In this regard, the concave lenses are required to have high dispersion (i.e., a low Abbe number).
When the above-described optical transparent resins are considered from the viewpoint of high dispersion (low Abbe number), the polycarbonate made of bisphenol A has a refractive index of about 1.59 and an Abbe number of about 32, the polymethyl methacrylate has a refractive index of about 1.49 and an Abbe number of about 58, and the amorphous polyolefin has a refractive index of about 1.54 and an Abbe number of about 56. Among them, only the polycarbonate can be used as concave lenses for aberration correction, but when the Abbe number is 32, it cannot be said that sufficiently high dispersion is obtained thereby. For this reason, a novel material which can be used as concave lenses for aberration correction has been desired.
As a resin to be used for concave lenses for aberration correction, Patent Document 1 discloses a polyester resin composition obtained by copolymerization of a fluorene-based dihydroxy compound having a refractive index of about 1.66 and an Abbe number of about 20.
Next, birefringence will be described. The polycarbonate resin made of bisphenol A is widely used for optical lenses, but applications of the polycarbonate resin are limited because of high birefringence thereof as a drawback. In particular, in applications to cameras for cellular phones and digital cameras, as the resolution has been increased recently by the improvement of the pixel number, a resin material having high imaging performance and low birefringence has been desired.
Examples of methods for realizing low birefringence of resin materials include a technique of canceling positive birefringence of a composition with negative birefringence of another composition (Patent Document 5). The sign (positive or negative) of birefringence is determined by the difference between the polarizability of the polymer main chain direction and the polarizability of the polymer side chain direction. For example, a polycarbonate resin made of bisphenol A in which the polarizability of the polymer main chain direction is larger than the polarizability of the polymer side chain direction has positive birefringence, and a polycarbonate resin made of bisphenol having a fluorene structure in which the polarizability of the polymer side chain direction is larger has negative birefringence. For this reason, the component ratio of materials whose birefringence signs are opposite to each other is very important. By using a resin having low birefringence, optical distortion is reduced.
Patent Document 6 describes that use of a dicarboxylic acid having a fluorene structure as a raw material in a polyester resin is effective for reduction of birefringence. Note that carboxylic acid is a type of a compound having a hydroxyl group (Non-Patent Document 1).
Note that polymers having a 1,1′-binaphthalene structure are described in Patent Documents 2, 3 and 4. However, Patent Documents 2 and 3 do not disclose any resin having a structural unit derived from a compound represented by general formula (1). Patent Document 4 describes a polymer comprising a structural unit represented by general formula (A), but it is not a polymer comprising a structural unit having a fluorene structure. Further, the patent document does not disclose whether the sign of birefringence of the polymer comprising a structural unit represented by general formula (A) is positive or negative.
(X represents a C1-10 alkylene group.)
(In the formula, A represents a C2-4 alkylene group. The naphthalene ring may be substituted with a substituent, and substituted substituents may be subjected to ring condensation. m and l respectively represent an integer of 0≤m≤50 and an integer of 0≤l≤50.)